An advertisement program attempts to select advertisements to display on a given web page based on how likely a user is to click on the advertisement. Selecting a relevant advertisement based on the content of a web page is one way to increase the likelihood of a user of the web page clicking on the advertisement. Several factors are considered in determining the relevancy of an advertisement to the content of a web page. One factor relates to keywords, which are words or phrases that are associated with advertisements and web pages. An advertiser may, for example, select keywords for an advertisement through an online keyword auction. Keywords may be selected for a web page by, for example, a content analysis engine or a web page owner. An advertisement whose keywords are closely matched with the keywords of a web page may have an increased likelihood of being clicked on. Therefore, a closely matched advertisement has a high likelihood of being selected by the advertisement program to be displayed on a given web page.
Advertisers strive to select a group of keywords that will increase the likelihood of their advertisement appearing on a web page. They frequently monitor the success of their keyword strategy and continue to improve their keyword selection to maximize their revenues. Advertisements may be matched to a web page using all the keywords associated with the advertisement. As a result, it is difficult for advertisers to monitor the effect a single keyword has on their advertisement being selected and clicked on by a user.